Rotating screens are known for use in many applications including municipal sewage and sludge treatment and food processing. Typically conventional screening devices have a hollow screening cylinder which is journalled for rotation about a horizontally oriented axis. Influent to be screened is fed into the interior of the rotating cylinder onto the screening surface. Filtrate from the influent then passes under gravity through openings in the screening surface, leaving behind solids which are too large to move through in the screens. As the cylinder rotates, solids retained internally within the cylinder are moved along the interior of the cylinder to a discharge outlet.
Conventional screening devices suffer the disadvantage in that the separation of the solid portions of the influent from the filtrate is achieved solely as a result of the aperture size and the screening surface. Practically therefore, there is a need to strike a balance between having a cylinder screen with a mesh size which is so large as to permit large quantities of solid portions to readily move through the screen openings, and a cylinder screen with a mesh size which is so small as to become clogged or otherwise hinder drainage of the filtrate.
The use of rotary screening devices in the pulp and paper industry to increase the solid content of wood pulp has also been proposed. Typically, however, such conventional screening surfaces are formed having wedge wire screens, as for example is described in U.S. Pat. No. 3,876,548. Wedge wire screens are formed from a plurality of parallel screen bars which have an outer face and angled sides. Wedge wire screens have been found to be inefficient in separation on removal of liquids as fluid therein tends to flow as a sheet across the flat inward facing wire surfaces, rather than through the screen opening, as the screening drum rotates. In addition to being inefficient in separation of liquids from solids, wedge wire screens have been found particularly unsuitable for use in treating wood pulp, as elongate pulp fibres tend to collect within and clog openings in the screens, preventing the proper outward movement of filtrate from the cylinder and necessitating repeated cleaning of the apparatus increasing down time.